1. Field of the Invention
The present invention relates to a process whereby ethylene polymer resins having broad molecular weight distributions obtained from chromium catalyzed polymerizations are improved by modifying with peroxide. By contacting the resins with peroxide, the top load stress crack resistance of blow molded bottles produced therefrom is significantly improved.
2. Description of the Prior Art
In the selection and production of polyethylene resins for the manufacture of blow molded goods, a careful balance must be struck between the physical properties and processing characteristics of the resin if efficient production of durable molded articles is to be achieved. While many ethylene polymers have superior physical properties, they do not have acceptable theological properties under conditions of flow and shear such as are encountered during the blow molding operation. Conversely, other resins which exhibit satisfactory rheological behavior are deficient in one or more essential physical characteristics. There is a continuing effort to develop resins which have an optimal balance of physical and rheological properties, especially high density polyethylene resins useful for the manufacture of blow molded bottles.
Improvement in the processing characteristics i.e., flow properties and shear response, of high density polyethylene (HDPE) produced using supported chromium catalysts has been obtained by broadening the molecular weight distribution of the polymer. This can be accomplished by blending separately prepared polymers of different molecular weights or by utilizing catalyst systems capable of directly producing polymers having broadened molecular weight distributions.
U.S. Pat. No. 4,025,707, for example, discloses the preparation of high density ethylene homopolymers and copolymers of broadened molecular weight distribution utilizing a mixed catalyst comprised of several portions of the same or different chromium components and metal promoted variations thereof in which the various portions are activated at different temperatures and thereafter combined. Chromium catalysts consisting of a mixture of first and second chromium-containing catalyst components obtained using silica supports of differing pore volumes and wherein one of the catalyst components also contains aluminum and a polymerization process utilizing same to produce HDPE resins having good processability and resistance to environmental stress cracking are disclosed in U.S. Pat. No. 5,081,089. It will also be recognized by those skilled in the art that, for a given supported chromium catalyst, molecular weight distribution can be varied through the use of various co-catalytic agents and modifiers in the polymerization and by varying the support and/or activation conditions used to prepare the catalyst.
While it is possible to broaden molecular weight distribution using chromium catalysts and processes such as those described above, any processing advantages have been accompanied by a corresponding decrease in one or more essential physical properties. For example, while the products obtained in accordance with U.S. Pat. No. 4,025,707 have good swell characteristics and flow properties, densities are generally considered to be too low to provide adequate stiffness for blown bottles. The HDPE polymers obtained using the mixed chromium catalysts of U.S. Pat. No. 5,081,089 have high densities and blow molded bottles produced from these resins have good stiffness, column crush properties (determined by ASTM D 2659-89) and environmental stress crack resistance (determined by ASTM D 2561). However, resistance to stress cracking when the bottles are placed under top load is unsatisfactory.
Environmental stress crack resistance (ESCR) and column crush do not provide a reliable indication of performance of filled blown bottles placed under top load, a condition encountered when bottles are stacked for storage, shipping or display. The Plastic Bottle Institute, a Division of the Society of the Plastics Industry, Inc., has accordingly developed a test to simulate these conditions and determine top load stress crack resistance (TLSCR) of blown polyolefin bottles. The test, identified as PBI 11-1978, Rev. 1-1991, determines resistance to stress cracking under constant top load conditions at elevated temperatures. The purpose of this test is to compare one group of bottles to another of the same design under the same top load and conditions.
It would be highly advantageous if high density polyethylene resins suitable for blow molding produced in chromium catalyzed particle form polymerization processes having an optimal balance of processability and physical properties were available. It would be even more advantageous if bottles produced using the high density resins had improved resistance to stress cracking under top load and if such improvement could be achieved without adversely affecting other essential physical properties and while further improving the processing characteristics of the resin. These and other advantages are achieved with the present invention wherein resins with specified molecular weight distribution, vinyl unsaturation content and long chain branching index are modified by contacting with an organic peroxide and which will be described in greater detail to follow.
Treatment of polyethylene resins with peroxides is known. Peroxides have been widely used to crosslink polyethylene and, in other instances, to "modify" the resins without substantial crosslinking. The latter procedures are generally distinguished from the former in that the modification is accomplished without the formation of gel. They are sometimes referred to in the literature as "lightly crosslinking" or "coupling" to distinguish them from true crosslinking reactions where a substantial amount of material insoluble in boiling xylene is produced.
While the melt index of virtually every polyethylene resin is reduced upon treatment with peroxide, it is not possible to accurately predict what effect such treatment will have on other rheological behavior or on the physical properties of the polymer. Reported results vary significantly from resin to resin even when the resins are produced using similar catalyst technology.
Polyethylene resins produced using Ziegler-Natta catalysts are treated with low levels of free radical initiators in U.S. Pat. No. 4,603,173 to lightly crosslink and thereby improve processing without substantially changing physical properties. The resins used have specific molecular weight characteristics obtained by combining a low molecular weight component with a very high molecular weight component. A reduction in flare swell and die swell is effected without substantially changing weight swell and physical properties.
U.S. Pat. No. 4,390,666 also discloses crosslinking different Ziegler resins to produce a different result. High and medium density polyethylene of high molecular weight and high and medium density polyethylene of low molecular weight are melt blended and lightly crosslinked. Die swell, stiffness, impact and ESCR are improved. Linear medium or low density Ziegler-produced polyethylene is similarly modified by uniformly melt kneading with a low level of radical initiator in the process of U.S. Pat. No. 4,465,812. In yet another reference, U.S. Pat. No. 4,508,878, Ziegler-produced HDPE blow molding resins are pelletized in the presence of peroxide and oxygen to increase die swell and improve surface appearance.
Treatment of polyethylene resins prepared using other catalyst with peroxides is also known and produces widely divergent results. In U.S. Pat. No. 2,993,882 peroxide treatment is employed with polyethylene and ethylene copolymers to decrease the melt index by at least half while the solid state properties of the treated product are essentially unchanged. Copolymers of ethylene and higher alpha-olefin having a density of less than 0.935 are treated with organic peroxides in the process of U.S. Pat. No. 4,460,750 to improve the transparency of films produced therewith but without significantly affecting mechanical properties.
U.S. Pat. No. 4,578,431 discloses improving the melt strength of LLDPE and HDPE film by peroxide treatment without substantially affecting the other desirable film properties.
Ethylene polymer blends wherein the mixture or one of the components in the mixture contains an organic peroxide are disclosed in U.S. Pat. Nos. 4,614,764; 4,737,574 and 4,840,996.